As it is known, magnetic grain-oriented sheets are used mainly for manufacturing of electric transformer cores.
Commercially available products are classified based on magnetic properties thereof (as defined according to UNI EN10107 rule).
Such magnetic characteristics are associated with special product crystalline structure displaying an anisotropic crystallographic texture ({110} <001>) and macroscopic grain size (from mm to cm).
In order such structures to be obtained it is necessary particularly long, complex and very expensive industrial manufacturing cycles to be carried out, high degree of process control being further required. For all the degrees but particularly for thinner thicknesses (i.e. <0.30 mm) and higher B800 products, both physical and magnetic process yields are particularly critical parameters resulting in a meaningfully incidence on product cost.
All current technologies for manufacturing of grain-oriented magnetic sheet take advantage of the same metallurgical strategy in order to obtain the extremely strong Goss texture for final sheets, that is the process for secondary oriented re-crystallization assisted by second and/or segregating phase distribution. Second not metallic phases and segregating agents play a critical role for control (slowing down) of grain boundary movement during final annealing step by addressing orientation selective secondary re-crystallization process.
For example according to EP 0125653, EP 098324, EP 0411356 inhibiting elements are mainly manganese sulfide and aluminum nitride (MnS+AlN).
The above described technology, however, results in a drawback deriving from inheritance of slab microstructure, displaying large grains generated during solidification process.
These grains, because of reduced mobility of grain boundary resulting from alloy silicon occurrence, preventing complete re-crystallization during the process, lead to microstructure heterogeneities in turn resulting in that within final product zones wherein the grain is fine and not subjected to a correct secondary crystallization (said streaks) occur thus leading to impaired magnetic characteristics.
Recently new steel casting technologies aiming to have still more compact, flexible and further reduced cost production processes have been developed. An innovative technology advantageously used for the production of transformer sheets is thin slab casting characterized by continuous casting of long pieces directly to typical thicknesses of conventional blank bars and well suited to embodiment of direct rolling processes by coupling in continuous sequence slab casting, passage in continuous tunnel furnaces for heating of casted pieces and finishing rolling to wound strips. Casting at reduced thickness limits the whole amount of applied mechanical deformation for hot rolling, which in turn results in higher incidence of above described drawback. The persistence of not re-crystallized zones is one of main problems referred to manufacturing technologies starting from thin slabs.
All the technologies for industrial production of grain-oriented magnetic sheet based on slab or ingot casting, share that thickness reduction starting from casted slab or ingot to thin strip (final product) is carried out by a first hot rolling and then a second cold rolling with hot reduction rates ranging from 90% to 99% and typically lower total cold reduction rates (85-90%).
Many technologies in order to improve the amount and homogeneity of strip hot re-crystallization for manufacturing of said steels on the base, for example, of particular hot rolling conditions, have been proposed. Among most recent thereof, for example in WO2010/057913 a process wherein slabs are hot rolled by adjusting temperature and blanking reduction grade according to bar temperature over time range from blanking and finish rolling, is described. In US2008/0216985A1 a special cycle for strip hot manufacturing by applying high deformation rate at first stand of finishing train is described. In EP 2147127 hot rolling process wherein it is not necessary casted slab to be heated before rolling and first hot rolling step is carried out at temperature lower than slab core, is described.
According to the present invention when cold deformation is applied without strip hot annealing, a particular micro structural strip homogeneity is obtained thus avoiding drawback resulting from grain size heterogeneity within annealed cold rolled steel and presence of streaks within final product.
As it is well known by those skilled in the art, moreover, the elimination of strip hot annealing step in production cycle represents firstly an opportunity in order to reduce the manufacturing costs (i.e. energy costs, productivity and physical yield increases) to put into effect whenever possible, although a preliminary cold rolling treatment for surface conditioning purpose by a continuous surface sand-blasting process and/or acid pickling is considered necessary in order scale/oxidation material resulting from hot rolling to be removed from strip surface, is considered necessary. In methods involving strip hot annealing typically both the processes (annealing and pickling continuous lines) are carried out on same lines.